Method for producing an impeller of an exhaust gas turbocharger and tial alloy for an impeller

ABSTRACT

A method for producing an impeller of an exhaust gas turbocharger made of TiAl alloy by centrifugal casting, includes providing a TiAl alloy consisting of titanium, 43.7 at-% to 47.5 at-% aluminum, 1.9 at-% to 8.7 at-% niobium, 0.3 at-% to 0.6 at-% carbon, and at most 2.0 at-% others, heating a casting mold used for the centrifugal casting to a casting temperature of 500° C. to 800° C.; introducing the TiAl alloy into the heated casting mold; accelerating the casting mold about a rotation axis spaced apart from the casting mold with an angular acceleration to a rotational speed within an acceleration time period as soon as the TiAl has reached the casting temperature, wherein a rotary axis of the produced impeller is arranged perpendicular to the rotation axis during the centrifugal casting.

The invention relates to a method for producing an impeller of an exhaust gas turbocharger and a TiAl alloy. The invention also relates to a TiAl alloy.

TiAl alloys have a low density of about 4 g/cm³ and good specific high-temperature properties. Their use as material of the impeller of the exhaust gas turbocharger leads to a significantly improved response behavior of the exhaust gas turbocharger compared to known nickel-based alloys and offers a high potential with regards to downsizing or downspeeding. In Diesel or Otto internal combustion engines, the impeller of the exhaust gas turbocharger is impinged with temperatures of up to 1,050° C. and rotational speeds of up to about 220,000 rpm. At the same time the impeller is hereby subjected to high mechanical and thermal alternate stresses. This results in creep stress, TMF-, HCF- and/or LCF fatigue stresses, which the material, i.e., the TiAl alloy, has to be able to withstand. Further it also has to have a sufficient oxidation resistance under the mention high exhaust gas temperatures and has to be resistant against erosion.

In order to take full advantage of the potential for reducing the mass of the impeller and thus the moment of inertia by using the TiAl-alloy, no compromise regarding the geometric complexity and the number of impeller blades of the impeller must be made. Identical or smaller wall thicknesses and identical or stronger interleave of the blades of the impeller have to be able to be realized, as compared to impellers that are made from the nickel-based alloy with known methods. In known impellers made of TiAl alloys, however, the full potential of such an alloy cannot be realized because it has a lower flowability than the nickel-based alloy. In particular only simple geometries with blade wall thicknesses of the impeller of 1 mm or greater can be realized. Also the used TiAl-alloys have a comparatively low creep resistance or service life and can no longer be used at exhaust gas temperatures of 950° C. or more, in particular 980° C. or more or 1000° C. or more.

It is therefore an object of the invention to provide a method for producing an impeller of an exhaust gas turbocharger made of a TiAl alloy with which these disadvantages can be avoided.

According to the invention this is accomplished with a method with the features of claim 1. Hereby it is provided that the alloy has a content of at least 1.9 at % niobium and the impeller is formed by centrifugal casting of the alloy. The addition of Niobium within the stated substance amounts leads to a high creep resistance, in particular also at high temperatures of at least 950° C., at least 980° C. for at least 1000° C. At the same time, however, it leads to poorer casting properties, in particular to a further deteriorated flowability, so that a processing of the TiAl alloy with the conventional differential pressure casting is not possible. Preferably the Niobium content is at least 2 at.-%, at least 2.5 at.-%, at least 3 at.-%, at least 3.5 at.-%, at least 4 at.-%, at least 5 at.-%, at least 6 at.-%, at least 7 at.-% or at least 8 at.-%. Further preferably the Niobium content is at most 8.7 at.-%. Particularly preferably the niobium content is between 3 at.-% and 5 at.-%, in particular between 3.5 at.-% and 4.5 at.-%, further preferably at exactly 4 at.-%.

Surprisingly, however, it was found that by using centrifugal casting, fine or complex structures of the impeller, for example a small blade wall thickness, can nevertheless be realized in spite of adding niobium. In the centrifugal casting, a casting mold used for this purpose is caused to rotate and during the rotation the alloy is introduced into the casting mold. Due to the inertia moments acting on the alloy as a result of the rotational movement, a particularly preferred structure of the solidified alloy is achieved, in particular on the side of the formed impeller that faces away from a rotation axis of the casting mold. The term structure in particular means a microstructure and the surface of the impeller. The produced impeller has, in particular at the mentioned sites, a high purity, fewer pores, fewer shrinkage cavities, fewer cold shuts and fewer undesired surface reactions with the mold shell or the casting mold. This leads to a low roughness of the impeller surface and/or to a significantly improved strength, in particular creep strength. Due to the rotational movement, the inertia forces cause a high casting pressure, with which the alloy is forced into the casting mold. Correspondingly also complex structures and small blade wall thicknesses can be achieved without problems. The mentioned inertia forces are for example centrifugal forces and/or coriolis forces.

The centrifugal casting is in particular performed so that the casting mold is spaced apart from the rotation axis in radial direction (relative to the rotation axis). This means that the casting mold does not rotate about a later rotary axis of the produced impeller, but rather this rotary axis is for example perpendicular to the rotation axis. By correspondingly selecting the distance between the rotation axis and the casting mold, the inertia forces acting on the alloy or the impeller to be produced can thus be influenced. By means of such a method, impellers can be produced which at least in the region of the blades have very small wall thicknesses, for example at most 0.5 mm.

A refinement of the invention provides that a casting mold used for the centrifugal casting is heated to a temperature of 400° C. to 900° C. prior to introducing the alloy into the casting mold. In the stated temperature range between 400° C. to 900° C. (these values are respectively inclusive) the interlamellar spacing and the colony size are almost constant for a selected, solidified TiAl alloy. In this range almost constant microstructures with almost identical properties are thus achieved. Particularly preferably the temperature—which can also be referred to as casting mold temperature—is selected from a range of 500° C. to 800° C., in particular between 600° C. and 700° C. The casting mold is to be completely heated to the selected temperature, before the alloy is introduced into the casting mold.

Usually at the point of being introduced the alloy will have a temperature or casting temperature, which is greater than the temperature of the casting mold. Insofar there is a great difference or a great temperature gradient in the region of an inner walling of the casting mold that is in contact with the alloy. Such a great difference is required in order to be able to achieve a directional solidification of the alloy also in thin-walled elements of the impeller, in particular its blades. A prerequisite for this is beside the complete filling of the casting mold an under-peritektically solidified TiAl alloy. When these conditions are satisfied the lamellar colonies or the lamellas orient themselves during the solidification significantly in the direction of the temperature gradient or the wall thickness, i.e., in the direction of the normal to a surface of the blades.

Because during operation of the impeller the resulting inertia forces are also oriented perpendicular to the blade wall thickness or to the lamella colonies, the described orientation of the colonies leads to an improved creep resistance. Also the fast solidification caused by the great temperature difference leads to a pronounced forced solubilization of the carbon, which is preferably contained in the TiAl alloy. In response to stress, in particular during operation of the exhaust gas turbocharger, this solubilized carbon precipitates in the form of carbides, which impede dislocations and minimize creep deformation. Thus a significant improvement of the structure of the impeller is achieved. The alloy can thus contain an amount of carbon, which would normally be disadvantageous. In particular a content of carbon in the alloy of 0.2 at.-%, in particular at least 0.6 at.-%, in particular at least 0.3 at.-%, at least 0.4 at.-% or at least 0.5 at.-%.

A further advantageous embodiment of the invention provides that at the point the alloy is introduced into the casting mold, the alloy has a casting temperature, which is overheated by 40K to 150 K relative to the liquidus temperature. This means that the casting temperature is higher than the liquidus temperature by the value of the alloy is overheated. In particular the casting temperature is selected so that it is overheated by a value in the range between 42 K and 142 K (with these values being respectively inclusive). Preferably the overheating is at least 92 K, in particular exactly 92 K. Varying the overheating within the mentioned range of 40 K to 150 K only leads to a slight change of the heat flow dissipated over the surface of the impeller. The influence of the temperature of the casting mold therefore outweighs the influence of the casting temperature with regard to the heat flow dissipated by the alloy to the casting mold.

Overall the temperatures in the stated range therefore only slightly change the solidification time of the alloy. Within the stated temperature range the solidification time is within an optimal solidification time range. Insofar the above-described superior properties are achieved with such a casting temperature. In order to achieve a complete filling of the casting mold with the alloy also in the case of impellers with wall thicknesses of at most 0.5 mm, in particular at the blade tip of the impeller, the overheating of at least 92 K is used.

A further embodiment of the invention provides that during the centrifugal casting the casting mold is accelerated about a rotation axis spaced apart from, the casting mold, to a defined rotational speed with a defined angular acceleration within a defined acceleration time, as soon as the alloy has reached the casting temperature. In order to perform the centrifugal casting, the casting mold is thus caused to rotate about the rotation axis. As described above, the casting mold is hereby spaced apart from the rotation axis. The acceleration occurs with the defined angular acceleration, wherein the casting mold is brought to the defined rotational speed within the defined acceleration time period. Preferably the acceleration is only initiated when the alloy has reached its casting temperature. It can be provided that the casting mold stands still beforehand. For example it is provided that due to the rotational movement the alloy is forced into the casting mold. In particular it is provided that the influence of the inertia force is sufficient to cause the alloy to exit the storage container in which it was heated to the casting temperature beforehand, and to introduce the alloy into the casting mold with a defined casting pressure.

Hereby it can be provided that the angular acceleration is between 1 s⁻² to 100 s⁻², in particular 1 s⁻² to 10 s⁻², or from 10 s⁻² to 100 s⁻². For a complete filling of the casting mold it is advantageous when the filling process is completed prior to, or respectively with, completion of the acceleration. For this reason an angular acceleration is particularly preferably selected, that leads to a filling time that is as short as possible. A high angular velocity, however, also leads to quickly reaching the defined rotational speed or a final rotational speed, above which the casting mold is no longer accelerated. For example the angular acceleration is greater than 1 s⁻² and smaller than 100 s⁻², in particular greater than 1 s⁻² and smaller than 10 s⁻² or greater than 10 s⁻² and smaller than 1 s⁻². The stated values can also be enclosed by the value range delimited by these values.

After reaching the defined final rotational speed, however, the mass flow of the alloy introduced into the casting mold significantly decreases and leads to a lower filling pressure, in particular when the casting filling is not already completed at this time point. This may lead to an increased number of casting errors. An increase of the final rotational speed merely has a small influence on the mass flow or the achieved mold filling, when the mold filling is complete prior to reaching the defined rotational speed. The final rotational speed, however, together with the distance of the casting mold to the rotation axis determines the casting pressure. Thus because a too low final rotational speed is disadvantageous, a relatively high final rotational speed is selected in the following description. The term mold filling in particular means the proportion of the casting mold already filled with the alloy.

A preferred embodiment of the invention provides that the acceleration time period is between 0.05 s and 2 s, in particular from 0.5 s to 2 s. The acceleration time period is in particular selected so that the selected rotational speed is reached by using the selected angular acceleration. As mentioned above the acceleration time period, which results from these parameters, can have a duration so that the casting process is completed at the expiration of the acceleration time period, i.e., the casting mold is completely filled.

A further embodiment of the invention provides that the rotational speed is between 100 rpm and 500 rpm. As mentioned above a too low rotational speed is disadvantageous for the casting pressure because the rotational speed together with the distance between the rotation axis and the casting mold is important for the casting pressure. For this reason the rotational speed is selected form the stated range between 100 rpm and 500 rpm (these values are respectively inclusive).

Particularly preferably it can be provided that the distance between the casting mold, in particular a geometric center of gravity of the casting mold, and the rotation axis is between 200 mm and 1500 mm. With such a distance, in particular together with using a rotational speed selected from the above stated rotational speed range, a casting pressure is achieved that leads to a complete filling of the casting mold and/or to a particularly advantageous microstructure of the produced impeller and/or enables a particularly small blade wall thickness of 1 mm or less, in particular 0.5 mm or less. The term distance between the casting mold can be understood as the distance between a point of the casting mold which in radial direction is located furthest inwardly or a point of the casting mold which in radial direction is located furthest outwardly on one hand and the rotation axis on the other hand. Particularly preferably, however, this term means the distance between the geometric center of gravity of the casting mold and the rotation axis. The term casting mold for example means the mold cavity in which the impeller is formed.

It can be provided that the angular acceleration is selected so that it is greater than the result of the division of a value between 100 mm²/s² and 300,000 mm²/s², in particular a value of 100 mm²/s², and the product of a blade wall thickness of the impeller and the distance of the casing mold to the rotation axis. The term blade wall thickness of the impeller hereby for example means a minimal blade wall thickness or alternatively a maximal or average blade wall thickness of the impeller. The angular acceleration is expressed in rad/s². Preferably the angular acceleration is selected greater than the result of the division of a number between 100 and 300,000 (respectively inclusive these values), in particular a number from 100 to 1,000, and the product of the blade wall thickness in millimeters and the distance of the casting mold to the rotation axis in millimeters.

In addition it can be provided that the rotational speed is selected to be greater than the product of a value between 0.04 1/min and 50 1/min and the ratio of the distance of the casting mold to the rotation axis and the blade wall thickness of the impeller. The two last mentioned values are preferably defined according to the descriptions above. The rotational speed or the final rotational speed—in the unit revelations per minute—is for example greater than the product of a number between 0.4 and 500 (inclusive these values) and the ratio of the distance of the casting mold to the rotation axis in centimeters and the blade wall thickness of the impeller, in particular the average blade wall thickness of the impeller, in millimeters.

It can also be provided that the length of the acceleration time period is selected so that it at most corresponds to the product of a value between 0.1 s/mm and 20 s/mm, and the blade wall thickness of the impeller. The duration of the target acceleration time period stated in seconds thus corresponds at most to the product of a number from 0.1 to 20 and the blade wall thickness in millimeters. Particularly preferably the length of the acceleration time period corresponds to the filling time period of the casting mold. In any case the length of the acceleration time period corresponds at least to the filling time period.

Finally it can be provided that the distance between the casting mold and the rotation axis is selected to be greater than or equal to the product of a number between 100 and 5,000 and the blade wall thickness of the impeller. Expressed in centimeters, the distance between the casting mold and the rotation axis is thus greater than or equal to the product of the number from 10 to 500 (inclusive these numbers) and the blade wall thickness of the impeller in millimeters.

Preferably the casting mold is evacuated prior to introducing the alloy. Correspondingly an under pressure is present in the casting mold relative to the ambient pressure. The absolute pressure in the casting mold is for example at most 1 mbar, in particular at most 0.1 mbar, particularly preferably at most 0.05 mbar or at most 0.01 mbar. The evacuation counteracts an embrittlement of the impeller, which could otherwise occur due to the affinity of the alloy to oxygen.

The invention also relates to a TiAl alloy for an impeller of an exhaust gas turbocharger, in particular produced with the method described above, wherein the TiAl alloy beside Titanium contains the following components:

-   -   Aluminum with a content of 43.7 at-% to 47.5 at-%     -   Niobium with a content of 1.9 at-% to 8.7 at-%,     -   Carbon with a content of 0.3 at-% to 0.6 at-% and     -   Others with a content of at most 2.0 at-%.

The advantages of the TiAl and of the method were described above. The TiAl and the method for producing the impeller can be refined according to the description above so that insofar reference is made to this description. The described alloy can of course also be used for the method described above. This also applies to the following described alloys.

In order to find the optimal alloy composition for the impeller, numerous alloys were tested. The optimal content of aluminum was found to be the range from 43.7 at-% to 47.5 at-%. Below an aluminum content of 44.8 at-% the solidification path extends via the beta phase. An aluminum content of at least 44.8 at-% to less than 47.3 at-% leads to an under-peritectic solidification path, while at an aluminum content of 47.3 at-% a peritectic solidification path is given. An aluminum content of more than 47.3 at-% on the other hand leads to a super-peritectic solidification path. The alloys within the aluminum content according to the invention of 43.7 at-% to 47.5 at-% (inclusive these values), which solidify under-pertectically, show better creep properties compared to alloys that solidified via the beta phase. The under-peritectically solidified alloys are therefore preferred, in particular the aluminum content is exactly 45.8 at-%.

The following sated compositions are purely exemplary:

-   -   Alloy 1: 45 at-% aluminum, 3.7 at-% niobium, 0.25 at-% chromium         and 0.5 at-% carbon.     -   Alloy 2: 45.8 at-% aluminum, 3.9 at-% niobium, and 0.3 at-%         carbon.     -   Alloy 3 45.8 at-% aluminum, 3.9 at-% niobium, and 0.6 at-%         carbon.     -   Alloy 4 47.0 at-% aluminum, 8.7 at-% niobium, and 0.3 at-%         carbon.

The remaining components of the stated alloys are titanium and other components, for example impurities, with a content of at most 2 at-%.

The aluminum content can alternatively also be expressed by way of the aluminum equivalent. In the binary TiAl phase diagram, the upper and lower Al limit for an under-peritectic solidification are at 44.8 at-% and 47.3 at-%. As a result of the alloying with niobium and/or carbon, the limits are shifted and can correspondingly be specified by way of an aluminum equivalent. Niobium shifts the limits to higher values and carbon to lower values. Therefore the respective element content is multiplied with a calculation factor and correspondingly subtracted from or added to the limits. The calculation factor for niobium is 0.3, and for carbon −4.2.

The minimal aluminum content Al_(min) is 44.8 at-%, the maximal aluminum content Al_(max) is 47.3 at-%. The lower limit for the aluminum equivalent results for the smallest stated niobium content of 1.9% and the greatest stated carbon content of 0.6 at-%. From this the lower limit for the aluminum equivalent results as 44.8 at-%+1.9 at-%·0.3-0.6 at-%·4.2=42.85 at-%. The upper limit for the aluminum equivalent on the other hand results from the greatest niobium content of 8.7 at-% and a smallest carbon content of 0.0 at-% according to 47.3 at-%+8.7 at-%·0.3-0.0 at-%·4.2=48.8 at-%. Alternatively the TiAl alloy is thus characterized in that it contains aluminum, niobium and carbon with an aluminum equivalent of 42.85 at-% to 48.8 at-%.

Particularly preferably the alloy consists exclusively of the stated components; i.e., no further components or impurities are contained.

In the following the invention is explained in more detail by way of the exemplary embodiments shown in the drawing, without limitation. The sole FIGURE shows a schematic representation of a casting device for producing an impeller of an exhaust gas turbocharger made of a TiAl alloy.

The FIGURE shows a device 1, in particular a centrifugal casting device for producing a not further shown impeller of an exhaust gas turbocharger. The device 1 has a storage container 2 for a TiAl alloy 3 and a casting mold 4. The storage container 2 and also the casting mold 4 are arranged in a chamber 5 of a casting mold carrier 6. The casting mold carrier 6 is fastened on an arm 7 and is supported by the arm for rotation about a rotation axis 8. The rotation is directed for example in the direction indicated by arrow 9. Preferably on the side facing away from the casting mold carrier 6 in relation to the rotation axis 8 a compensation weight 10 is fastened on the arm 7. The device 1 also has a heating coil 12, which can be displaced in the direction of the double arrow 11. Instead of the compensation weight 10 of course a further casting mold carrier can also be provided, which is preferably configured identical to the casting mold carrier 6. Thus parallel casting processes can be performed.

For preparing the casting process, the chamber 5 is preferably evacuated, in particular via a connection socket 13, wherein an evacuation of the air present in the chamber 5 occurs along the arrows 14. Also the casting mold 4 is preferably heated to a defined temperature between 400° C. and 900° C. In addition the alloy 3 contained in the storage container 2 is heated to a casting temperature by means of the heating coil 12, which is in particular constructed as a high-frequency coil. When the casting temperature is reached, the heating coil 12 is arranged so that it no longer surrounds the storage container 2, in particular by a downward movement. Then a rotation of the casting mold carrier 6 about the rotation axis 8 is initiated. During the centrifugal casting the casting mold 4 is thus rotated about the rotation axis 8, which is spaced apart from the casting mold, within a defined acceleration time period to a defined rotational, speed, as soon as the alloy has reached its casting temperature.

Due to the inertia force resulting from the rotation, the molten alloy 3 is forced radially outward (relative to the rotation axis 8). This means it is forced in the direction of the arrow 15 out of the storage container 2 and into the casting mold 4. The forcing into the casting mold 4 hereby occurs with a casting pressure, which is essentially influenced by a distance between the rotation axis 8 and the casting mold 4 on one hand and the actual rotational speed of the casting mold carrier 6 on the other hand. As a result of the spaced apart arrangement of the casting mold 4 from the rotation axis 8, a very high casting pressure can be achieved with such a centrifugal casting or centrifugal fine casting. This produces a very good casting result also in the case of TiAl alloys that cannot, or only with great effort, be processed with other casting methods. This applies in particular to a TiAl alloy which has a niobium content of at least 1.9 at-%.

By means of the centrifugal casting significantly finer structures in the form of small interlammellar spaces can be produced compared to differential pressure casting. This applies independent of the aluminum content and the niobium content of the TiAl alloy. The higher fineness of the microstructure results in the improved creep properties of the impeller made of the alloy compared to the differential pressure casting: In particular the impeller produced by the centrifugal casting can be used at temperatures, which are 50K higher than an impeller made by means of differential pressure casting.

LIST OF REFERENCE SIGNS

-   1 device -   2 storage container -   3 alloy -   4 casting mold -   5 chamber -   6 casting mold carrier -   7 arm -   8 rotation axis -   9 arrow -   10 compensation weight -   11 double arrow -   12 heating coil -   13 connecting socket -   14 arrow -   15 arrow 

1.-10. (canceled)
 11. A method for producing an impeller of an exhaust gas turbocharger made of a TiAl alloy by centrifugal casting, comprising: providing a TiAl alloy consisting of titanium, 43.7 at-% to 47.5 at-% aluminum, 1.9 at-% to 8.7 at-% niobium, 0.3 at-% to 0.6 at-% carbon, and at most 2.0 at-% others; heating a casting mold used for the centrifugal casting to a temperature of 500° C. to 800° C.; introducing the TiAl alloy into the heated casting mold; accelerating the casting mold about a rotation axis spaced apart from the casting mold with an angular acceleration to a rotational speed within an acceleration time period as soon as the TiAl has reached the casting temperature, wherein a rotary axis of the produced impeller is arranged perpendicular to the rotation axis during the centrifugal casting
 12. The method of claim 11, wherein the TiAl alloy when being introduced into the casting mold has a casting temperature, which is higher by 40K to 150K relative to a liquidus temperature of the TiAl alloy.
 13. The method of claim 11, wherein the angular acceleration is between 1 s⁻² to 100 s⁻²,
 14. The method of claim 11, wherein the angular acceleration is from 1 s⁻² to 10 s⁻² or
 15. The method of claim 11, wherein the angular acceleration is from 10 s⁻² to 100 s⁻².
 16. The method of claim 11, wherein the acceleration time period is between 0.05 s and 2.0 s,
 17. The method of claim 11, wherein the acceleration time period is from 0.5 s to 2.0 s.
 18. The method of claim 11, wherein the rotational speed is between 100 rpm and 500 rpm.
 19. The method of claim 11, wherein a distance between the casting mold and the rotation axis is between 200 mm and 1500 mm. 